The field of molecular electronics has developed rapidly in the last 15 years with the discovery of organic conductive and semi-conductive compounds. During this period, a large number of compounds exhibiting semi-conductive or electro-optical properties have been found. As well as oligothiophenes, thiophene-phenylene co-oligomers, for example, are also important representatives of the semi-conductive compounds. Compounds of this type are currently of great interest for areas of application such as organic field effect transistors (OFETs), organic light-emitting diodes (OLEDs), sensors and photovoltaic elements.
The production of thiophene-phenylene co-oligomers, also referred to below as thiophene-phenylene oligomers, can take place by various coupling reactions known to the person skilled in the art. The fact that the suitability of the end products for use in semiconductor technology depends substantially on their purity, and the disadvantageous effects of contaminated semiconductors on the properties, such as e.g. a poor on/off ratio in OFETs, of electronic applications produced therefrom, are generally known (Handbook of Oligo- and Polythiophenes, ed. by D. Fischou, Wiley-VCH, Weinheim, pp. 469–471).
Hotta et al. describe a series of phenylene oligothiophenes and their production via Grignard reactions or Suzuki coupling, but products are formed that are so strongly contaminated that high losses of yield are recorded after purification (J. Heterocyclic Chem. 2000, 37, 281–286; J. Heterocyclic Chem. 2000, 37, 25–29, Synt. Met. 1999, 101, 551–552). In all cases, the Suzuki coupling takes place between aryl boronic acids and thienyl halides. The suitability of oligothiophenes with phenyl units as terminal groups as light-emitting coating materials in OLEDs was described by Hotta et al. in Synt. Met. 1999, 106, 39–40.
Katz et al. recently described other thiophene-phenylene oligomers and their suitability as oligomeric semiconductors for OFETs. Unfortunately, here too, the most suitable, 1,4-bis(5-hexyl-2,2′-bithien-5-yl)phenyl, could only be produced in an extremely low yield of 10% by coupling of organotin thiophene compounds with dihaloaryl compounds, and so this method is unsuitable for the production of larger quantities of oligomers (Chem. Mater. 2001, 13, 4686–4691). Katz et al. also describe the synthesis of 1,4-bis(2,2′-bithien-5-yl)phenyl and 1,4-bis(4-hexyl-2,2′-bithien-5-yl)phenyl by Suzuki coupling of corresponding thiophene-phenylene-thiophene dihalides with monothiopheneboronic acids. However, it is a disadvantage of this method that a central thiophene-phenylene-thiophene dihalide unit has to be produced by complex coupling reactions. Moreover, thiopheneboronic acids are credited with low stability (Gronowitz et al., Heterocycles 1990, 30, 645–658; Lehn et al., Eur. Chem. J. 1996, 2, 1399–1406). Both result in high losses of yield and/or larger quantities of impurities.
In US-A 2002/0114973, other thiophene-phenylene oligomers are described, but the disadvantage still exists there that, with an increasing chain length, increasing numbers of coupling steps are needed to construct the molecule, which in turn leaves the problem of the high loss of yield and larger quantities of impurities, which have to be eliminated by means of complex, high-loss purification. US-A 2002/0114973 also describes the production of thiophene-phenylene oligomers by Grignard reactions and/or Suzuki coupling using thiopheneboronic acid.
A need exists, therefore, for a process for the production of semi-conductive organic thiophene-phenylene oligomers with which the end products are obtained in high purity and good yield via the fewest possible intermediates. Such a process for the production of these oligomers should advantageously be applicable regardless of their chain length.